I am not a P275 owner, but I have a P27 which I suspect may originally have had the RM69. toilet installed. with most of these small toilets their footprint is often similar, so I would first measure the base where your existing toilet is installed and if possible, make a cardboard template of it with an old cardboard box. take it to your local Jabsco/Johnson toilet distributor or chandlery and offer it up, the staff will I am sure be happy to help. If new holes need drilling, make sure to fill the old holes to maintain the surface integrity of the plinth, you don't want "spills" making their way into the void below, which will cause unpleasant smells! There should be an access panel below the plinth to get at the nuts that hold down the toilet. The hoses should be similar sizes and are not an issue if they are different as you can fit reducers as necessary but they should be the same or very similar and the hose material will be able to accommodate small changes. They are usually 3/4"/19mm and 2"/50mm inlet and outlet fittings. be sure to double clip all the hoses as you are attaching fittings below the waterline and a failure of a lone clip could spell disaster. it can all be done whilst the boat is in the water so long as you have ( or should have) through-hull valves which obviously need shutting before removing the hoses. To facilitate removal and fitting of the hose, use a hot air gun to warm the hose around the fitting to soften it, be careful not to burn the hose as that will weaken it and could cause localised hardening which could then split. It is probably a good idea to replace the sanitary hoses whilst you are at it as over time the old hose age hardens and is very had to reroute. Be sure to replace with good quality white sanitary hose, don't be tempted to use any other hose as you will end up with smells permeating the hose which is not pleasant. it is not a particularly difficult job except for the fact you are working in a small restricted space, and depending on you age and flexibility, may be a challenge! I am 76 and suffer from arthritic joints but I changed my toilet over to a composting head last year which required rebuilding/reglassing the plinth and the heads space is a lot smaller on a P27 than on a P275! Good Luck!